1. Field of the Invention
The present invention relates to a thin film-forming apparatus used in the manufacture of semiconductor devices.
2. Description of the Related Art
Semiconductor devices are improving remarkably. Their integration density is becoming higher, with rapid progress in miniaturization of the entire device and in reduction in thickness of the thin films used. In the meantime, however, a problem arises in that if foreign particles are incorporated into a thin film, this may cause defects in the pattern of the thin film which in turn may lead to a reduction in reliability.
FIG. 1 is a view showing the overall structure of a conventional thin film-forming apparatus. This apparatus is a vertical-type atmospheric pressure chemical vapor deposition apparatus (hereinafter simply referred to as an "atmospheric pressure CVD apparatus"). Referring to the drawing, the apparatus includes a vessel 1 defining a reaction chamber A, a gas dispersion head 2 which is mounted on the upper portion of the vessel 1 and through which gases for forming a thin film are supplied to the reaction chamber A, gas supply pipes 3 connected to the gas dispersion head 2, a stage 4 provided on the lower portion of the vessel 1 and in opposition to the gas dispersion head 2, and a heater 5 provided within the stage 4. Reference numeral 6 designates a substrate to be processed by forming a thin film thereon. The substrate 6 is, for instance, a wafer formed of single crystal silicon. The apparatus further includes an exhaust port 7 formed in the peripheral portion of the bottom face of the vessel 1. The flow of gases within the reaction chamber A is indicated by arrows 8.
Next, an explanation will be given concerning a method of forming, for instance, a silicon oxide film, using this atmospheric pressure CVD apparatus.
First, the wafer 6 is conveyed into the reaction chamber A through a conveyance port (not shown) and is then placed upon the stage 4. At this time, the stage 4 already has its temperature set at a predetermined temperature of, e.g., about 350 to 450.degree. C., by the action of the heater 5.
Subsequently, gases used to form a thin film are supplied from a gas supply source (not shown) into the gas supply pipes 3, thereby introducing the gases into the reaction chamber A through the gas dispersion head 2. If a silicon oxide film is to be formed, the gases used are reaction gases such as monosilane (SiH.sub.4) gas and oxygen (O.sub.2) gas, and a carrier gas such as nitrogen (N.sub.2) gas. The monosilane (SiH.sub.4), oxygen (O.sub.2), and nitrogen (N.sub.2) gases used in this example are supplied into pipes 3a, 3b, and 3c, respectively, forming the gas supply pipes 3. The gases are then supplied in their mixed state through an unillustrated nozzle portion of the gas dispersion head 2 toward the wafer 6 disposed below.
A part of the reaction gases, etc., within the reaction chamber A is exhausted to the outside through the exhaust port 7 located on the periphery of the stage 4. At this time, the remaining part of the gases within the reaction chamber A forms a flow, such as that indicated by the arrows 8 in FIG. 1, whereby the reactive gases are evenly supplied to the surface of the wafer 6. As a result, the reaction expressed by the following formula takes place on the surface of the wafer 6: EQU SiH.sub.4 +O.sub.2 .fwdarw.SiO.sub.2 +2H.sub.2
A silicon oxide film is formed on the surface of the wafer 6 in this way.
However, since the gases supplied fill the entire reaction chamber A, the result is that while a silicon oxide film 12 is formed on the surface of the wafer 6, as shown in FIG. 2, silicon oxide molecules also adhere to the inner wall surface 1a of the vessel 1. Silicon oxide molecules adhering to the inner wall surface 1a gradually grow as the film-forming operation is repeated, resulting in the formation of a deposit 11. As this deposit 11 grows, some of it may peel off from the inner wall surface 1a, and some of the peeled-off deposit may even adhere to the surface of the wafer 6 being processed. Furthermore, since the reaction which results in the generation of silicon oxide also takes place in the atmosphere above the wafer 6, the resultant silicon oxide molecules may cohere to form particles 9. These particles 9 may adhere to the surface of the wafer 6 being processed. In this way, foreign particles 10 may adhere to the surface of the wafer 6 being processed and may cause defects in the silicon oxide film 12 being formed. Thus, it has been difficult to form high-quality thin films with a high yield.